1. Field of the Invention
The present invention relates to an X-ray exposure method using fine pattern formation technology and a semiconductor device manufactured using this X-ray exposure method as well as an X-ray mask, an X-ray exposure unit and a resist material, in particular, to an X-ray exposure method and a semiconductor device manufactured using this X-ray exposure method as well as an X-ray mask, an X-ray unit and a resist material wherein finer pattern transcription is made possible in comparison with a prior art having a primary objective of the use of fine patterns on the surface of a mask in a system wherein a fine pattern manufactured on a mask is transcribed by means of X-ray proximity exposure technology with respect to transcription technology used primarily in semiconductor integrated circuit manufacture.
2. Description of the Background Art
FIG. 15 schematically shows the conventional X-ray proximity exposure method. X-rays 1 emitted from an electron storage ring 10 are condensed to a predetermined range by a mirror 2, are transmitted through an X-ray vacuum protection filter 3, and irradiate a wafer 6, having a resist 5 attached thereto, arranged in proximity to an X-ray mask 4 so as to have a predetermined gap from this X-ray mask 4. The pattern of X-ray mask 4 is formed of X-ray absorber 4a and X-ray transmitting body 4b, and the X-rays that have been transmitted through X-ray absorber 4a and X-ray transmitting body 4b irradiate resist 5 on wafer 6. Secondary electrons are generated by resist 5 that has absorbed X-rays 1 and the X-ray mask pattern is transcribed onto resist 5 because of a chemical change caused in resist 5 due to those secondary electrons.
The resolution of X-ray proximity exposure is determined by two different factors. That is to say, the resolution limit of an optical image is determined according to Fresnel diffraction and the resolution limit is determined by so-called secondary electron fuzziness due to the spread of photoelectrons or Auger electrons, generated in a resist due to irradiation with exposure light, to a limited region. The higher the energy of electrons, the greater the fuzziness of secondary electrons and the lower the energy of the electrons, the smaller the fuzziness of secondary electrons. The shorter the X-ray absorption wavelength becomes, the higher the energy of photoelectrons from among secondary electrons becomes so as to lower the resolution.
On the other hand, the absorbed X-ray energy image formed in the resist is determined by the Fresnel diffraction of the X-rays transmitted through the X-ray mask and the resolution limit R thereof is expressed in the following equation.R=k(λ·G)1/2
Here, k is a constant dependent on the mask absorber material, the mask pattern form as well as the exposure system. λ represents the X-ray wavelength absorbed by the resist and G represents the gap between the mask and wafer. It is understood from this equation that, given the same resist and same exposure method, the shorter is the X-ray wavelength, or the smaller is the mask-wafer gap, the greater the resolution becomes and the longer is the X-ray wavelength, or the greater is the mask-wafer gap, the poorer the resolution becomes.
However, as for X-rays used for actual exposure, emitted light having a continuous spectrum that is emitted from electron storage ring 10, X-rays emitted from a plasma X-ray source having a narrow spectrum width that is comparatively close to that of a single wavelength and X-rays emitted from various other X-ray sources are used. Therefore, the resist absorbs X-rays of various wavelengths emitted from the X-ray source. Then, the sum of the absorbed energy images formed according to the respective wavelengths becomes the absorbed energy image in the resist.
Accordingly, in the shorter wavelength side of the exposure wavelength range, primarily fuzziness of secondary electrons in the resist increases and, in the longer wavelength side, primarily resolution is lowered due to Fresnel diffraction. Therefore, in the case of the use of X-rays 1 emitted from electron storage ring 10, which is the X-ray source, short wavelength components are reduced by primarily utilizing the point that the shorter is the wavelength, the lower is the reflectance of mirror 2. Moreover, long wavelength components are reduced by utilizing the point that the longer is the wavelength, the greater is the absorbed X-ray ratio of X-ray vacuum protection filter 3 and of X-ray transmitting body 4b of X-ray mask 4. Thereby, exposure is carried out in a wavelength range wherein short wavelength components and long wavelength components that have a possibility of causing the lowering of resolution are reduced.
(First Problem to be Solved)
Here, the first problem to be solved by the present invention is the inclusion of X-rays having wavelengths that degrade the quality of the absorbed energy image in the X-rays absorbed by the resist.
As for the spectrum of X-rays absorbed by the resist obtained according to the conventional X-ray proximity exposure method, FIG. 16 shows two examples in the case wherein X-ray transmitting body 4b of X-ray mask 4 is made of silicon carbide (SiC) having a film thickness of 2 μm and wherein X-ray transmitting body 4b is made of diamond having a film thickness of 2 μm.
Here, an electron accelerating voltage of electron storage ring 10 of 0.685 GeV with a magnetic deflection of 3.5 T, mirror 2 being a platinum coated mirror, X-ray vacuum protection filter 3 made of beryllium having a film thickness of 18 μm and resist 5 being a PHS resist with a base resin of a polyhydroxystyrene (PHS, formula C8H8O1) are posited. As is shown in FIG. 16, in both cases wherein X-ray transmitting body 4b is of silicon carbide or diamond, a continuous X-ray in a range of approximately from 3 Å to 16 Å is absorbed by the resist.
In the case that the mask-wafer gap is constant and wherein the necessary resolution is set as Rr, if the resolution limit of all wavelengths of the X-rays absorbed by the resist 5 is higher than the necessary resolution Rr, then the necessary resolution Rr can be obtained without fail.
On the other hand, if X-rays of a wavelength lower than the resolution limit Rr are included among the X-rays irradiated on the surface of resist 5, the resolution of the absorbed energy image formed in resist 5 will be degraded.
Here, FIG. 17 shows an example of the contrast of the absorbed energy image absorbed in the resist according to respective X-ray wavelengths in regard to a 50 nm L and S (line and space) mask pattern, calculated in the range from 100 nm to 400 nm of a film thickness of a tungsten (W) X-ray absorber. The mask-wafer gap is 10 μm.
In the calculation, after calculating the absorbed image according to Fresnel diffraction, this is integrated with the relatively small Gaussian distribution σ=5 nm, as the fuzziness of an optical image and, thereby, the absorbed energy image was found. As for the contrast value, the amount of the resist absorption energy at the resist position directly beneath the center of the transmitting body pattern of the mask pattern is denoted as Imax, the amount of the resist absorption energy at the resist position directly beneath the center of the X-ray absorber pattern of the mask pattern is denoted as Imin so that the contrast was defined according to the following equation.contrast=(Imax−Imin)/(Imax+Imin)
In other words, the greater the contrast is above 0, the sharper is the absorbed energy image and the easier it is to form the pattern. The upper limit of the contrast is 1. On the other hand, when the contrast is 0 or less, the absorbed energy image is degraded because the amount of absorbed energy below the transmitting body becomes smaller than the amount of absorbed energy below the X-ray absorber.
It is understood in reference to FIG. 17 that, in the case of a 50 nm L and S mask pattern, although also dependent on the film thickness of the X-ray absorber, in general the contrast becomes 0 or less, so that the absorbed energy image is degraded in regard to X-rays having wavelengths from 6 Å to 7 Å and X-rays having wavelengths longer than 8 Å to 8.5 Å from among the continuous X-rays absorbed by the resist in the range of from approximately from 3 Å to 12 Å. Though the wavelength range wherein the contrast becomes 0 or less, changes according to dependence on the pattern dimensions, the mask structure and the mask-wafer gap, a problem can be cited wherein, as described above, an X-ray wavelength range that lowers the resolution is included among the of X-ray spectrum absorbed by the resist.
In particular, the wavelength range of from 6 Å to 7 Å corresponds to the vicinity of the X-ray absorption edge (6.9 Å) of tungsten, which is the material of the X-ray absorber, and the X-ray absorption coefficient, as well as the amount of X-ray phase shift, of the X-ray absorber fluctuate greatly. Therefore, the contrast of the absorbed energy image is lowered. Consequently, a problem can be cited that when the absorption edge of the X-ray transmitting body and of the X-ray absorber are in the exposure wavelength range, the contrast of the absorbed energy image in the wavelength range in the vicinity of their absorption edges is lowered.
Further, the problem can be cited that when the film thickness and density of X-ray transmitting body 4b of X-ray mask 4 and of X-ray vacuum protection filter 3 are increased in order to reduce long wavelength components, wavelength components that contribute to resolution arriving at the surface of resist 5 are also reduced and throughput is lowered.
(Second Problem to be Solved)
Next, the second problem to be solved by the present invention is the inclusion of X-rays that lower resolution as a result of Fresnel diffraction of X-rays transmitted through the mask.
First, in reference to FIG. 18, the X-ray phase conditions for obtaining a high contrast optical image will be described. In the X-ray mask pattern wherein X-ray absorber 4a and X-ray transmitting body 4b are lined up in alternation in a periodic manner, in order to obtain a high contrast optical image, it is required for the X-rays transmitted through X-ray transmitting body 4b and the X-rays transmitted through X-ray absorber 4a to be mutually reinforced at point P on the surface of the resist directly beneath X-ray transmitting body 4b and to be mutually weakened at point Q on the surface of the resist directly beneath X-ray absorber 4a. In order for mutual strengthening directly beneath X-ray transmitting body 4b, it is ideal for the phase difference of the X-rays that have been transmitted through X-ray transmitting body 4b and the X-rays that have been transmitted through X-ray absorber 4a to be 0 at point P, in other words, to be of equiphase. Further, in order for mutual weakening directly beneath X-ray absorber 4a, it is ideal for the phase difference of the X-rays that have been transmitted through X-ray transmitting body 4b and the X-rays that have been transmitted through X-ray absorber 4a to be π at point Q, in other words, to be of antiphase.
The phase difference between the X-rays that have been transmitted through X-ray transmitting body 4b and the X-rays that have been transmitted through X-ray absorber 4a can be separated into two. One is the phase difference due to the X-ray optical path difference determined based on the mask-wafer gap and the positional relationship on the mask and the other is the phase difference occurring when X-rays are transmitted through X-ray absorber 4a. If it is assumed that the phase difference between X-ray transmitting body 4b and X-ray absorber 4a is −π/2 when the phase difference due to the optical path difference becomes π/2, the X-ray phase conditions are achieved wherein a high contrast optical image is obtained according to the conventional X-ray proximity exposure method.
For example, the phase difference of an X-ray absorber 4a is posited as −π/2 and the phase difference of X-rays that have been transmitted between points B-Q in relation to X-rays that have been transmitted between points A-Q is posited as π/2. At this time, the X-ray phase difference also becomes π/2 for X-rays that have been transmitted between points B-P in relation to X-rays that have been transmitted between points A-P. Consequently, X-ray phase conditions are achieved wherein high contrast can be obtained in relation to X-rays that have passed the center of the pattern.
Next, problems with this method are explained. As described above, only X-rays that are transmitted through the center portion of the pattern of X-ray transmitting body 4b and through the center portion of the pattern of X-ray absorber 4a fulfilled the ideal X-ray phase conditions but X-rays that are transmitted through positions shifted from the center portion can not necessarily be said to fulfill the ideal X-ray phase conditions.
Consider the phase difference due to the X-ray optical path difference of X-rays that have reached point Q having passed edge C of X-ray absorber 4a and X-rays that have reached point Q having passed edge D of X-ray transmitting body 4b. When the distance from point A to point C as well as the distance from point A to point D is L/2 and 3L/2, respectively, the phase difference due to the optical path difference for X-rays that have been transmitted between points C-Q and X-rays that have been transmitted between points D-Q is expressed in the following equation (1).((3L/2)2−(L/2)2)/2/G/λ×2π=2·L2/2/G/λ×2π  (1)
Here, G is the mask-wafer gap and λ is the wavelength of the incident X-rays.
Furthermore, the phase difference due to the optical path difference for X-rays that have passed between points B-Q in relation to X-rays that have passed between points A-Q is expressed in the following equation (2).L2/2/G/λ×2π=π/2  (2)
When equation (2) is substituted in equation (1) the phase difference due to the optical path difference becomes π.
Consequently, it is clear that this deviates from the X-ray phase condition π/2 wherein a high contrast can be obtained. Therefore, the problem of lowering of resolution due to irradiation by X-rays deviating from optimum X-ray phase conditions such as these can be cited.
Further, as for a method to obtain resist pattern dimensions smaller than the pattern dimensions of an X-ray transmitting part of a mask pattern using Fresnel diffraction, an X-ray transmitting part with pattern dimensions approximately two times wider than the necessary resist pattern dimensions is used. Therefore, a problem can be cited wherein the smallest pitch among the resist pattern becomes more than two times wider than the resist pattern dimensions and an increase in integration of semiconductor circuits becomes difficult.